1. Field of the Invention
The present invention relates to a wavelength-division multiplexing (WDM) communication system.
2. Description of the Related Art
Due to the rapid progress of optical wavelength multiplexing technology and its introduction to the market, rapid market expansion of backbone networks has been promoted in recent years. The number of wavelength-division multiplexed waves of a WDM system has exceeded 160 and the number of devices connected to the WDM system, such as synchronous optical networks (SONET), synchronous digital hierarchy (SDH) devices and the like has greatly increased. As a result, installation space and cost have exponentially increased as the number of wavelengths to be multiplexed increases, and the burden on communication carriers has increased. In such a situation, optical networks for converting enormous optical signals multiplexed by a WDM function into electrical signals in each node and distributing them to each path cannot meet the market demand any more with respect to the processing speed and cost of the devices. Therefore, demand for optical routers, optical cross-connect devices and the like, capable of processing wavelength-divisionally multiplexed (WDM) optical signals without any additional processing, for example, routing them as light, has been increasing, and the development of technologies related to it has been urgently required.
FIG. 1 shows the basic configuration of a conventional optical cross-connect.
Since an optical cross-connect switches routes light as it is, there is no need to terminate electrical signals. When a WDM optical signal which is wavelength multiplexed one is inputted to an optical cross-connect, a wavelength demultiplexer 10 demultiplexes the optical signal into a plurality of optical signals each with one of wavelengths λ1 through λn. The demultiplexed optical signals are inputted to an optical switch structure after being demultiplexed. Then, after being switched, the optical signal is outputted for each wavelength again and is inputted to a wavelength multiplexer 11. The wavelength multiplexer 11 applies wavelength-division multiplexing (WDM) to the optical signals with each wavelength, and generates/outputs a WDM optical signal.
FIG. 2 shows a conventional WDM system.
As shown in FIG. 2, in the prior art, an optical supervisory channel (OSC) control signal is provided separately from a WDM optical signal, and the OSC control signal stores information for switching light with each wavelength. When receiving this OSC signal, each optical cross-connect station determines the switching direction of optical switching, based on the contents of the OSC signal and performs the optical switching. However, since in such a method, optical switching information must be put on the OSC control signal, the trace information of SONET/SDH device or the like for outputting each wavelength must be transferred to an OSC signal transmitting unit provided for a WDM device. Therefore, such information must be transmitted/received between all SONET/SDH devices 15 or transponder devices 16 (a device for converting received light into light with a predetermined wavelength in order to apply WDM to it to transmit it), connected to the WDM device.
In the above-mentioned prior art, such signal information of SONET/SDH device or the like to be put on each wavelength channel must be taken in by converting it from optical to electrical in the WDM device once or a circuit line which communicate the information should be provided by the other means.
Specifically, in the transponder device 16, such as the SONET/SDH device 15 or the like, control information stored in the header of data formed in a frame is once demultiplexed for each frame without any conversion by the photo-coupler 18 of the WDM station (WDM device) 17, is terminated at a data communication channel (DCC) termination/OSC transmission unit 19 and is converted into an electrical signal. Then, necessary control information is extracted from the electrical signal and is converted into an optical signal with an OSC channel wavelength. This OSC channel optical signal is multiplexed onto a main signal transmitted from a WDM device 20 in the latter stage of the WDM device 20 for multiplexing optical signals from the SONET/SDH device 15 or the like, by a coupler 21.
The WDM optical signal generated thus is transmitted through a transmission line and is inputted to an optical cross-connect station 25. Upon receipt of the WDM optical signal, the optical cross-connect station 25 demultiplexes an OSC channel from the received WDM optical signal in an OSC separation unit 22, and converts the OSC optical signal into an electrical signal in an OSC termination/transmission unit 23. Then, the OSC termination/transmission unit 23 extracts control information from the electrical signal and issues a switching instruction to an optical switch structure 12. Then, this control information is converted into an optical signal again and is inputted to an OSC insertion unit 24 as an OSC optical signal.
The WDM optical signal from which the OSC signal is separated is demultiplexed into signals with each wavelength by a wavelength demultiplexer 10, and is switched by the optical switch structure 12. Then, after WDM is applied to the demultiplexed signals in a wavelength multiplexing unit 11, the OSC signal is inserted in the multiplexed signal by the OSC insertion unit 24 and is transmitted from the optical cross-connect station 25.
FIG. 3 explains the conventional problems.
Conventionally, a system in which switching information and the like is converted from optical to electrical and is collected as the number of wavelengths increases, as shown in FIG. 3, increases cost. Specifically, in a WDM station 17, an optical signal with each wavelength is transmitted to a DCC termination unit 19-1 after being once converted into an electrical signal, and an OSC signal is generated in an OSC transmission unit 19-2. Therefore, the number of optical/electrical converters 30 becomes enormous. Namely, in a WDM system, the number of multiplexed wavelengths is considered to exceed 100 in the future. However, in the configuration shown in FIG. 3, since an optical/electrical converter 30 must be provided for all wavelengths exceeding 100, 100 or more optical/electrical converters 30 become necessary and hardware increases.
If a communication device, such as a SONET/SDH device or the like provided for each channel is installed in the same station as the WDM device as a separate communication means, there is no need for optical/electrical conversion, since electrical signals can be used. Therefore, a simple and inexpensive communication method for transmitting/receiving such information can be realized. However, if a communication device, such as a SONET/SDH device or the like is not installed in the same station as the WDM device, communication must be conducted by light as long as connection is made only by an optical fiber. In that case, cost increases.